FIG. 17 is a general configuration diagram illustrating electrochemical machining equipment known in the art, which is described, for example, in Japanese Patent Disclosure Publication No. 216723 of 1989. FIG. 18 is a sectional view showing a vertically emphasized machined surface profile of a workpiece machined by the electrochemical machining equipment. In FIGS. 17 and 18, the electrochemical apparatus for electrochemical machining a workpiece 1 comprises a tool electrode 2, a machining tank 3, a machining liquid 4, a tool electrode drive motor 5 for driving a drive mechanism 6 which converts the rotary motion of the drive motor 5 into the vertical motion of the tool electrode 2, a machining liquid supplier 7 providing machining liquid 4, a machining liquid supplying solenoid valve 8, a machining liquid supplying injection nozzle 9, a machining liquid rejection hole 10 in the tool electrode 2, an electrolytic current power supply 11 for supplying a pulse current between the tool electrode 2 and workpiece 1, and a controller 12 for controlling the position of the tool electrode 2 relative to the workpiece 1. The controller 12, which also controls the magnitude and waveform of the supplied pulse current and the amount of the machining liquid 4 supplied comprises an electrode position control section 13 for outputting a command signal to the drive motor 5 to make the gap value between the workpiece 1 and tool electrode 2 equal a predetermined gap, a machining liquid control section 14 for outputting a control signal to the machining liquid supplier 4, a machining condition control section 15 for setting the peak current value, pulse width and pulse cycle of the supplied pulse current and outputting a command signal to the electrolytic current power supply 11 corresponding to a plurality of set values. An input device 16 for entering the set values is externally coupled to controller 12.
FIG. 19 illustrates the electrolytic current power supply 11 and machining condition control section 15 in detail. The electrolytic current power supply 11 comprises a direct current power supply 70, and a charger/discharger 71. The direct current power supply 70, in turn, consists of a transformer 72 for converting an input voltage to a predetermined voltage and a rectifier 73 for rectifying the predetermined voltage. The resultant direct current is output to a plurality of accumulators 74-1 to 74-n which will be described in greater detail below.
The charger/discharger 71 comprises a plurality of accumulators 74-1 to 74-n for discharging electric charges to the machining gap, diodes 75-1 to 75-n connected to corresponding ones of the accumulators 74-1 to 74-n for inhibiting reverse flow of the electric charges toward the direct current power supply 70, discharging switches 76-1 to 76-n which are turned on for discharging the electric charges to the discharge side, and a charging switch 77 for turning power from the direct current power supply 70 on and off to charge the accumulators 74-1 to 74-n according to a predetermined operating sequence.
The machining condition control section 15 of FIG. 19 comprises a voltage detector 78 for detecting the voltage values of the accumulators 74-1 to 74-n, a voltage comparator 80 for comparing a voltage value detected by the voltage detector 78 with a desired output value provided by a digital-to-analog converter 79 and a charge detector 81 for detecting the completion and start of charging of accumulators 74-1 to 74-n in accordance with the output signal of the voltage comparator 80. Control section 15 further comprises a current detector 82 for detecting the current values of the electric charges discharged to the machining gap, a peak holding circuit 83 for holding the peak value of the current value detected by the current detector 82 and a current comparator 85 for comparing the peak current value held by the peak holding circuit 83 with the output value of the digital-to-analog converter 84. Additionally, control section 15 comprises a gate circuit 88 for outputting an on/off drive signal to the discharging switches 76-1 to 76-n in accordance with input signals from a pulse generator 86 which generates a pulse of predetermined time width and from a current waveform setter 87 which sets the current waveform of the electric charges discharged to the machining gap, a charge voltage setter 89 for setting a charge voltage value supplied to the accumulators 74-1 to 74-n and outputting the signals thereof to the digital-to-analog converter 79 and a current setter 90 for setting a current value flowing in the machining gap and outputting the signal thereof to the digital-to-analog converter 84. An electric charge discharger 91 for discharging the electric charges residing in the accumulators 74-1 to 74-n, an electric charge discharging command device 92 for outputting a control signal to the electric charge discharger 91, a contact detector 93 for detecting contact of the tool electrode 2 and the workpiece 1, a central processing unit (CPU) 94 for performing operations and processing of machining conditions in accordance with the input and other data provided through the input device 16 and other components are also included in control section 15. As shown in FIG. 19, diodes 95 are provided for preventing the discharging switches 76-1 to 76-n from being damaged by back electromotive force.
The operation of the electrochemical machining unit configured as illustrated in FIGS. 17 and 19 will now be described. First, the command value set to the electrode position control section 13 in the controller 12 causes the drive motor 5 to operate and control the position of the tool electrode 2 via the drive mechanism 6 so that the tool electrode 2 is opposed to the workpiece 1 across a predetermined machining gap in the machining tank 3. Then the electrolytic current power supply 11 and the machining condition control section 15 shown in the FIG. 19 are operated as described referring to the flowchart in FIG. 20.
After securing the workpiece 1 and the tool electrode 2 in predetermined positions, the unit power supply 11 is turned on at step 100. Then at least one key, for example, a discharging key, (not shown) of the input device 16 is pressed. This causes the discharging switches 76-1 to 76-n to be turned to the on state and a control signal to be output from the electric charge discharging command device 92 to the electric charge discharger 91. During step 101, this signal switches off the electric charge discharger 91 which was switched on at the preceding step 100.
When the electric charge discharger 91 is switched off, the lengths of times T1 to Tn, during which a predetermined current can flow in resistors comprising the electric charge discharger 91, are read from the CPU 94 during step 102. The times T1 to Tn are periods of time which make the average power consumption of the resistors constant when the electric charges are discharged via the resistors of the electric charge discharger 91. T1 is defined as the period of time equivalent to area S1, which is power shown in FIG. 21A, when the current can flow at the maximum charging voltage of the accumulators 74-1 to 74-n. T2 to Tn are defined as the lengths of time when an area identical to the area S1 of T1 is provided for areas S2 to Sn, respectively. The periods of time T1 to Tn are calculated in advance from a discharge characteristic according to the static capacities C of the accumulators 74-1 to 74-n and the resistance values R of the resistors in the electric charge discharger 91, and are then stored in the CPU 94.
A predetermined cycle time T, for instance, a time longer than time Ts required for discharging a predetermined voltage Vs, which is discussed in greater detail below, is then calculated during step 103 in accordance with T1 to Tn. Voltages V1 to Vn corresponding to T1 to Tn, which are prestored like the values T1 to Tn and having the characteristic shown in FIG. 21B , are then read out of CPU 94 during step 104. When the voltages V1 to Vn have been read, step 105 is executed and the voltage detector 78 detects a plurality of voltages Vo, which are the residual electric charges of the accumulators 74-1 to 74-n, in response to the control signal of the machining condition control section 15 and determines whether the voltage Vo is less than the predetermined value Vs during step 106. When the result is NO, i.e. if the voltage Vo of the accumulators 74-1 to 74-n is greater than Vs, voltage Vi, which is greater than Vo, and time Ti corresponding to the voltage Vi are selected during step 107 from among the values V1 to Vn read at step 104. The electric charges are then discharged for the period of time Ti during step 108. After waiting for a time T-Ti to elapse during step 109, the operation returns to the preceding step 105 and the above operation is repeated until the result is YES at step 106. In other words, the voltage Vo, i.e. residual electric charge, of the accumulators 74-1 to 74-n is discharged by switching on the electric charge discharger 91 for lengths of time Ti+1, Ti+2, . . . in sequence at a constant cycle time T in accordance with the control signal of the electric charge discharging command device 92, until the voltage Vs, which allows a current equivalent to the specified power value W of the resistors in the electric charge discharger 91, is achieved. At this time, the discharging switches 76-1 to 76-n remain turned on.
If the result is YES in step 106 , i.e. the voltage Vo of the accumulators 74-1 to 74-n is less than Vs, the electric charges are discharged at step 110 for the predetermined period of time, e.g. time Ts longer than times T1 to Tn, the discharging of the residual electric charges is terminated, and the electric charge discharger 91 is switched on during step 111 by the control signal of the electric charge discharging command device 92.
When the discharge of electric charge stored in the accumulators 74-1 to 74-n before the start of machining, i.e., the charges stored at the termination of the preceding machining, is complete in the above sequence of processes and there are no electric charges left, machining is initiated. The electrode 2 first moves down and makes contact with the workpiece 1. When this contact is detected during step 112 by contact detector 93, the CPU 94 stores that point of contact as machining datum A and causes the machining liquid supplier 7 to operate to supply the electrolyte into the machining tank 3 during step 113. The CPU 94 then forces the electrode 2 to retract to position the electrode 2 in a position equal to the machining gap entered from the input device 16 during step 114.
When the electrode 2 opposes the workpiece across the machining gap and the electrolyte in the machining gap has "rested" during step 115, a predetermined pulse current corresponding to the machining area S of workpiece 1 is supplied by the accumulators 74-1 to 74-n in accordance with the control signal of the machining condition control section 15 during step 116, and the electrode 2 is raised during step 117 after that pulse current is switched off.
Fresh electrolyte is then injected during step 118 by the machining liquid supplier 7 from the injection nozzle 9 or the injection hole 10 to eliminate the electrolytic product in the machining gap that has been eluted by the pulse current supplied. The electrode 2 is then lowered during step 119 to make contact with the workpiece surface, and this position of contact is compared with the datum A by the CPU 94 to measure the depth of machining during step 120.
Step 121 causes steps 114 to 120 to repeat until the machining depth reaches the predetermined value. On reaching the predetermined depth of machining, the pulse current supplied by the accumulators 74-1 to 74-n is switched during step 122 to provide a predetermined pulse current by the control signal of the machining condition control section 15. Machining operations are repeated during steps 123 to 127, which are similar to those in steps 114 to 118 and which are repeated via step 128 a predetermined number of times. All machining operations are terminated at step 129 when a polished surface is provided.
It will be noted that during machining, the pulse current is continually controlled by the machining condition control section 15 and electrolytic current power supply 11 to keep the current density in the machining gap at the predetermined value.
The machined surface profile of the workpiece 1 machined by this electrolytic action is shown exaggerated in a vertical direction in FIG. 18. As illustrated in FIG. 18, the machined surface has been over- machined in the vicinity of the inner periphery and outer periphery of the tool electrode 2, resulting in a faulty machined profile. This is because, if the machining fluid 4 in the machining gap is still during the electrolytic action, more machining fluid 4 is supplied to the vicinity of the inner and outer peripheries of the tool electrode 2 due to heat convection, electrolytic bubbles, etc., leading to an uneven electrolytic action.
Configured as described above, the conventional electrochemical machining equipment is only controlled to provide a predetermined value of current density in the machining gap during machining. Accordingly, the equipment cannot operate appropriately to cope with the uneven response of the electrolytic action generated in the machining gap during electrochemical machining, producing a resultant faulty machined area.
In addition, to ensure predetermined surface roughness, the conventional electrochemical machining apparatus performs machining until the accumulation of machining depth values measured per supply of pulse current reaches a predetermined machining depth value By this method, accurate surface roughness cannot be provided without repeated data-based setting of a machined amount, and the machining depth cannot be measured accurately because electrolytic products exist in the machining fluid or on the tool electrode or workpiece. This measurement error does not allow the predetermined machining depth to be accurately determined thus over-machining causes a faulty product.